The viscosity of oil that will be sorbed in useful amounts by an oil sorbing mat is an important factor in judging the utility of the mat. The most successful and widely used oil sorbing materials are best used with medium-viscosity oils (10 to 1000 centipoises). When an attempt is made to use such materials with higher-viscosity oils (1000 to 100,000 centipoises), the reduced flowability of the oils interferes with the sorbing action of the oil-sorbing materials, and greatly limits the amount of oil that the materials will sorb.
The most successful commercial materials for sorbing high-viscosity oils are oil sorbing masses, on the order of 30 centimeters in diameter, of strips of polymeric film grouped and attached together like a pom pom. When such an oil sorbing mass is cast onto an oil spill, oil penetrates between and covers the strips, and a proportion of the penetrated oil is retained when the mass is removed from the spill.
However, such oil sorbing masses do not have the same efficiency as oil sorbing materials used on medium-viscosity oils. For example, when oil sorbing masses as described are used on oil spills of No. 5 fuel oil having a viscosity of about 6000 centipoises (i.e. at a temperature of about 24.degree. C), the sorbency number for the oil sorbing mass is about 15 (sorbency number is the weight of the sorbent after it has been immersed in oil for 5 minutes and then suspended in air and allowed to drain for 5 minutes; minus the weight of the sorbent when dry; divided by the weight of the sorbent when dry). By contrast, oil sorbents useful with medium-viscosity oils generally have sorbency numbers of 20 or more. The difference between 15 and 20 in sorbency number is important, since an oil sorbent having a sorbency number of 20 will sorb at least one-third again as much oil as an oil sorbent having a sorbency number of 15. The practical result of the inferior oil sorbency number is that clean-up operations for spills of high-viscosity oils require unsatisfactorily large amounts of sorbent material and unsatisfactorily long times to complete.